nand2/06_IO_Devices/UartRX.v

/**
 * UartRX receives bytes over UART
 *
 * When clear = 1 the chip clears the receive buffer and is ready to receive
 * next byte. out[15] is set to 1 to show, that chip is ready to receive next
 * byte. When RX goes low the chip starts sampling the RX line. After reading
 * of byte completes, chip ouputs the received byte to out[7:0]] with out[15]=0.
 */
`default_nettype none
module UartRX (
    input wire clk,
    input wire clear,
    input wire RX,
    output reg [15:0] out
);

    reg [7:0] baud_count;
    reg [3:0] bit_index;
    reg [7:0] rx_shift;
    reg rx_active;

    initial begin
        out <= 16'b0;
        rx_active <= 1'b0;
    end


    always @(posedge clk) begin
        
        out <= (clear | out[15]) ? 16'h8000 : out;

        if (!rx_active && !RX) begin

            // RX Falling edge: Start bit detected
            rx_active <= 1;
            rx_shift <= 0;
            baud_count <= 1;
            bit_index <= 0;
        end else if (rx_active) begin

            baud_count <= (baud_count == 216) ? 0 : baud_count + 1;
            bit_index <= (baud_count == 108) ? bit_index + 1 : bit_index;

            if (bit_index >= 1 && bit_index <= 8 && baud_count == 108) begin
                rx_shift <= {RX, rx_shift[7:1]};
            end

            if (bit_index == 10 && baud_count == 216) begin
                rx_active <= 0;  // Done receiving 8 bits + stop
                out <= {8'b0, rx_shift};
            end
        end
    end

endmodule